[PATCH v3 02/20] dcache: back inline names with a struct-wrapped array of unsigned long

Al Viro
2025-01-23T01:46:44Z

... so that they can be copied with struct assignment (which generates
better code) and accessed word-by-word.

The type is union shortname_storage; it's a union of arrays of
unsigned char and unsigned long.

struct name_snapshot.inline_name turned into union shortname_storage;
users (all in fs/dcache.c) adjusted.

struct dentry.d_iname has some users outside of fs/dcache.c; to
reduce the amount of noise in commit, it is replaced with
union shortname_storage d_shortname and d_iname is turned into a macro
that expands to d_shortname.string (similar to d_lock handling).
That compat macro is temporary - most of the remaining instances will
be taken out by debugfs series, and once that is merged and few others
are taken care of this will go away.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/dcache.c | 43 +++++++++-----------
include/linux/dcache.h | 10 ++++-
tools/testing/selftests/bpf/progs/find_vma.c | 2 +-
3 files changed, 28 insertions(+), 27 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index ea0f0bea511b..52662a5d08e4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -324,7 +324,7 @@ static void __d_free_external(struct rcu_head *head)

static inline int dname_external(const struct dentry *dentry)
{
- return dentry->d_name.name != dentry->d_iname;
+ return dentry->d_name.name != dentry->d_shortname.string;
}

void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
@@ -334,9 +334,8 @@ void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry
if (unlikely(dname_external(dentry))) {
atomic_inc(&external_name(dentry)->u.count);
} else {
- memcpy(name->inline_name, dentry->d_iname,
- dentry->d_name.len + 1);
- name->name.name = name->inline_name;
+ name->inline_name = dentry->d_shortname;
+ name->name.name = name->inline_name.string;
}
spin_unlock(&dentry->d_lock);
}
@@ -344,7 +343,7 @@ EXPORT_SYMBOL(take_dentry_name_snapshot);

void release_dentry_name_snapshot(struct name_snapshot *name)
{
- if (unlikely(name->name.name != name->inline_name)) {
+ if (unlikely(name->name.name != name->inline_name.string)) {
struct external_name *p;
p = container_of(name->name.name, struct external_name, name[0]);
if (unlikely(atomic_dec_and_test(&p->u.count)))
@@ -1654,10 +1653,10 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
* will still always have a NUL at the end, even if we might
* be overwriting an internal NUL character
*/
- dentry->d_iname[DNAME_INLINE_LEN-1] = 0;
+ dentry->d_shortname.string[DNAME_INLINE_LEN-1] = 0;
if (unlikely(!name)) {
name = &slash_name;
- dname = dentry->d_iname;
+ dname = dentry->d_shortname.string;
} else if (name->len > DNAME_INLINE_LEN-1) {
size_t size = offsetof(struct external_name, name[1]);
struct external_name *p = kmalloc(size + name->len,
@@ -1670,7 +1669,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
atomic_set(&p->u.count, 1);
dname = p->name;
} else {
- dname = dentry->d_iname;
+ dname = dentry->d_shortname.string;
}

dentry->d_name.len = name->len;
@@ -2729,10 +2728,9 @@ static void swap_names(struct dentry *dentry, struct dentry *target)
* dentry:internal, target:external. Steal target's
* storage and make target internal.
*/
- memcpy(target->d_iname, dentry->d_name.name,
- dentry->d_name.len + 1);
dentry->d_name.name = target->d_name.name;
- target->d_name.name = target->d_iname;
+ target->d_shortname = dentry->d_shortname;
+ target->d_name.name = target->d_shortname.string;
}
} else {
if (unlikely(dname_external(dentry))) {
@@ -2740,18 +2738,16 @@ static void swap_names(struct dentry *dentry, struct dentry *target)
* dentry:external, target:internal. Give dentry's
* storage to target and make dentry internal
*/
- memcpy(dentry->d_iname, target->d_name.name,
- target->d_name.len + 1);
target->d_name.name = dentry->d_name.name;
- dentry->d_name.name = dentry->d_iname;
+ dentry->d_shortname = target->d_shortname;
+ dentry->d_name.name = dentry->d_shortname.string;
} else {
/*
* Both are internal.
*/
- for (int i = 0; i < DNAME_INLINE_WORDS; i++) {
- swap(((long *) &dentry->d_iname)[i],
- ((long *) &target->d_iname)[i]);
- }
+ for (int i = 0; i < DNAME_INLINE_WORDS; i++)
+ swap(dentry->d_shortname.words[i],
+ target->d_shortname.words[i]);
}
}
swap(dentry->d_name.hash_len, target->d_name.hash_len);
@@ -2766,9 +2762,8 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
atomic_inc(&external_name(target)->u.count);
dentry->d_name = target->d_name;
} else {
- memcpy(dentry->d_iname, target->d_name.name,
- target->d_name.len + 1);
- dentry->d_name.name = dentry->d_iname;
+ dentry->d_shortname = target->d_shortname;
+ dentry->d_name.name = dentry->d_shortname.string;
dentry->d_name.hash_len = target->d_name.hash_len;
}
if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
@@ -3101,12 +3096,12 @@ void d_mark_tmpfile(struct file *file, struct inode *inode)
{
struct dentry *dentry = file->f_path.dentry;

- BUG_ON(dentry->d_name.name != dentry->d_iname ||
+ BUG_ON(dname_external(dentry) ||
!hlist_unhashed(&dentry->d_u.d_alias) ||
!d_unlinked(dentry));
spin_lock(&dentry->d_parent->d_lock);
spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
- dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
+ dentry->d_name.len = sprintf(dentry->d_shortname.string, "#%llu",
(unsigned long long)inode->i_ino);
spin_unlock(&dentry->d_lock);
spin_unlock(&dentry->d_parent->d_lock);
@@ -3194,7 +3189,7 @@ static void __init dcache_init(void)
*/
dentry_cache = KMEM_CACHE_USERCOPY(dentry,
SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_ACCOUNT,
- d_iname);
+ d_shortname.string);

/* Hash may have been set up in dcache_init_early */
if (!hashdist)
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 42dd89beaf4e..8bc567a35718 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -79,7 +79,13 @@ extern const struct qstr dotdot_name;

#define DNAME_INLINE_LEN (DNAME_INLINE_WORDS*sizeof(unsigned long))

+union shortname_store {
+ unsigned char string[DNAME_INLINE_LEN];
+ unsigned long words[DNAME_INLINE_WORDS];
+};
+
#define d_lock d_lockref.lock
+#define d_iname d_shortname.string

struct dentry {
/* RCU lookup touched fields */
@@ -90,7 +96,7 @@ struct dentry {
struct qstr d_name;
struct inode *d_inode; /* Where the name belongs to - NULL is
* negative */
- unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */
+ union shortname_store d_shortname;
/* --- cacheline 1 boundary (64 bytes) was 32 bytes ago --- */

/* Ref lookup also touches following */
@@ -591,7 +597,7 @@ static inline struct inode *d_real_inode(const struct dentry *dentry)

struct name_snapshot {
struct qstr name;
- unsigned char inline_name[DNAME_INLINE_LEN];
+ union shortname_store inline_name;
};
void take_dentry_name_snapshot(struct name_snapshot *, struct dentry *);
void release_dentry_name_snapshot(struct name_snapshot *);
diff --git a/tools/testing/selftests/bpf/progs/find_vma.c b/tools/testing/selftests/bpf/progs/find_vma.c
index 38034fb82530..02b82774469c 100644
--- a/tools/testing/selftests/bpf/progs/find_vma.c
+++ b/tools/testing/selftests/bpf/progs/find_vma.c
@@ -25,7 +25,7 @@ static long check_vma(struct task_struct *task, struct vm_area_struct *vma,
{
if (vma->vm_file)
bpf_probe_read_kernel_str(d_iname, DNAME_INLINE_LEN - 1,
- vma->vm_file->f_path.dentry->d_iname);
+ vma->vm_file->f_path.dentry->d_shortname.string);

/* check for VM_EXEC */
if (vma->vm_flags & VM_EXEC)
--
2.39.5